This invention relates to methods and systems for determining a position of a transceiver unit, such as employed on an aircraft, utilizing two-way ranging in a polystatic satellite configuration including a ground radar.
Current Automatic Dependent Surveillance (ADS) technology, such as Global Positioning System (GPS), Wide Area Augmentation System (WAAS) or GLONASS, provides positioning information utilizing satellite transmissions. For example, the GPS, developed and deployed by the U.S. Department of Defense, consists of 24 satellites orbiting the earth twice a day at an altitude of 12,000 miles, as well as five ground stations to monitor and manage the satellite constellation. Using atomic clocks and location data, GPS satellites transmit continuous time more satellites at once to determine the user""s position. By measuring the time interval between the transmission and the reception of a satellite signal, the GPS receiver calculates the distance between the user and each satellite, and then uses the distance measurements of at least three satellites to arrive at a position.
Such systems, however, utilize one-way ranging in which an accurate, synchronized clock is required at each station. Any synchronization error or error regarding the location of one of the satellites results in an error in the determined position of the target vehicle. Thus, there is a need to provide very accurate position and velocity information with a high degree of integrity and reliability.
It is thus a general object of the present invention to provide a method and system for determining a location of an object with a high degree of integrity and reliability utilizing two-way ranging in a polystatic satellite configuration to derive independent estimates of the transceiver""s state vectors including position and velocity.
In carrying out the above object and other objects, features, and advantages of the present invention, a method is provided for determining position of an object. The method includes the steps of transmitting a first ranging signal from a first known ground location to the object and transmitting a second ranging signal in response to the first ranging signal to the first known ground location. The method also includes the steps of transmitting a third ranging signal in response to the first ranging signal to a second known location and transmitting a fourth ranging signal in response to the third ranging signal to a third known location. The method further includes the step of determining a first delay corresponding to a time difference between transmission of the first ranging signal and receipt of the second ranging signal. The method also includes the step of determining a second delay corresponding to a time difference between transmission of the first ranging signal and receipt of the third ranging signal. Still further, the method includes the step of determining a third delay corresponding to a time difference between transmission of the first ranging signal and receipt of the fourth ranging signal. The method finally includes the step of determining the position of the object based on the first, second, and third known locations and the first, second and third delays.
In further carrying out the above object and other objects, features, and advantages of the present invention, a system is also provided for carrying out the steps of the above described method. The system includes a ground transceiver at a first known ground location for providing a bidirectional communication path between the ground transceiver and the object wherein the ground transceiver transmits a first ranging signal to the object and the object transmits a second ranging signal to the ground transceiver in response to the first ranging signal. The system also includes a first communication transceiver at a second known location for providing a first unidirectional communication path between the first communication transceiver and the object wherein the first communication transceiver performs one of transmitting a third ranging signal to the object and receiving a third ranging signal from the object in response to the first ranging signal. The system further includes a second communication transceiver at a third known location for providing a second unidirectional communication path between the second communication transceiver and the object wherein the second communication transceiver performs one of transmitting a fourth ranging signal to the object and receiving a fourth ranging signal from the object in response to the first ranging signal. Finally, the system includes a signal processor for determining a first path length corresponding to a first time length of the bidirectional communication path, determining a second path length corresponding to a second time length of the first unidirectional communication path, determining a third path length corresponding to a third time length of the second unidirectional communication path, and determining the position of the object based on the first, second, and third known locations and the first, second, and third path lengths.
The above object and other object, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.